Change the function signature - c++

Question

I'm new to C++ and for now there is one thing I would want to make clear. As I'm going through the tutorial,there is this program that stores user's input into an array and gives a sum of all numbers when the user exits the program:
//PROTOTYPE DECLARATION:
int readArray(int integerArray[], int maxNumElements);
int sumArray(int integerArray[], int numElements);
void displayArray(int integerArray[], int numElements);
int main(int nNumberofArgs, char* pszArgs[])
{
cout << "This program sums values entered\n";
cout << "Terminate the loop by entering a negative number" << endl;
//store the numbers from the user into a local array
int inputValues [128];
int numberOfValues = readArray(inputValues, 128);
//output the values and the sum of the values
displayArray(inputValues, numberOfValues);
cout << "The sum is " << sumArray(inputValues, numberOfValues) << endl;
return 0;
}
int readArray(int integerArray[], int maxNumElements)
{
int numberOfValues;
for(numberOfValues = 0; numberOfValues < maxNumElements; numberOfValues++)
{
//fetch another number
int integerValue;
cout << "Enter next number: ";
cin >> integerValue;
if (integerValue < 0)
{
break;
}
//otherwise store the number into the storage array
integerArray[numberOfValues] = integerValue;
}
//return the number of elements read
return numberOfValues;
}
//displayArray - display the members of an array:
void displayArray(int integerArray[], int numElements)
{
cout << "The value of the array is:" << endl;
for(int i = 0; i < numElements; i++)
{
cout << i << ":" << integerArray[i] << endl;
}
cout << endl;
}
//sumArray
int sumArray(int integerArray[], int numElements)
{
int accumulator = 0;
for(int i = 0; i < numElements; i++)
{
accumulator += integerArray[i];
}
return accumulator;
}
My questions are:
Is it neccessary to declare local variables in each function (e.g. int inputValues [128];)?
Would it be correct to store the input into the arguments that were declared in the function prototype? For example, can we just store everything into integerArray[] instead of creating a storage array integerValue ?
This may look obvious but I want to understand this to avoid making mistakes in the future.

Answer 1

inputValues is necessary if you want to pass an array to the function.
int inputValues [128];
int numberOfValues = readArray(inputValues, 128); //passing array to function
Either way you do it is fine. So what you have is not wrong.
As noted in the comments you could also pass inputValues by reference. Which you could declare the prototype of function like this.
int readArray(int (&integerArray)[128]);
Any changes you make to the array you passed by reference will be updated when the function returns to main because you are not operating on a copy of the array anymore.
Edit:
As #Kevin pointed out, you can use a template function to get the size of the array at compile time.
template<size_t N> int readArray(int (&integerArray)[N]);

Answer 2

There are a lot of understanding gaps in this question.
The function parameter lists will convert their input:
If the type is "array of T" or "array of unknown bound of T", it is replaced by the type "pointer to T"
Using the implicit array to pointer assignment:
Constructs a pointer to the first element of an array.
These two together hopefully help you to see that when you declare a function like: int readArray(int integerArray[], int maxNumElements), integerArray is really just a pointer to the first element of it's first argument. You call readArray(inputValues, 128) so the parameter integerArray is equivalent to &intputArray[0].

Answer 3

It is not necessary and you can use global variables instead but that is bad choice in terms security and visibility etc. This program can be done few different ways but I guess what you need to learn first is difference between local and global scope, pointer/array.
In the program, memory is allocated for
int inputValues[128]; //memory allocation
Then address of that location is passed here.
int numberOfValues = readArray(inputValues, 128);
It is much more efficient this way. But it will start make more sense once you get more experience with pointer and arrays.